What is the purpose of the Q3 transistor?

Thread Starter


Joined Jun 27, 2018
I have reversed engineered the stand-by/mute section of a PCB from a 2.1 computer speaker system. I understood and simulated the behavior of the circuit. The output of the simulation is as expected so I think I have got it right. So far so good.

However, if I remove the Q3 transistor the qualitative behavior of the circuit does not change at all. The only net result is that there is a couple of volts of difference between SUB_MUTE and SUB_STBY, which I think is not necessary at all for the functioning of the TDA7294 IC (but I could be wrong).

It is not clear to me what are the benefits of including the Q3 transistor. Could you please explain to me why they chose to put that transistor there?

I attached the schematics both as an image and as PDF. I attached also the output of the simulation with and without the Q3 transistor.